Volume 21, Number 3
Sep 1997

The Effects of Thymol on Paper, Pigments, and
Media

by Lisa Hall Isbell
(Conservator of Works of Art on Paper, Fairfax, VA)

[Note: This paper was originally given at the
Fourteenth Annual Art Conservation Training Programs Conference, May
4-6, 1988, at Buffalo State College. It appears in the 1989 volume
of "Papers Presented..." at that conference on p. 77-84, under the
author's maiden name, Lisa E. Hall.

The paper was one of three given in the session for student
papers the following year at the annual conference of the American
Institute for Conservation, but was not subsequently published. It
is reprinted here, with permission of the Buffalo State training
program and the author. Even though most readers are aware that
thymol has lost favor as a treatment for mold, they may appreciate
the detail in this technical report, because it helps explain why
thymol lost favor. -Ed.]

Abstract

This paper discusses the structure, uses, and hazards of thymol.
It then provides an overview of the record in conservation
literature of the effects of thymol on the paper support, pigments,
and media. It concludes with an experiment designed to test the
aging properties of works on paper exposed to thymol. From this
particular experiment, thymol appeared to have a deleterious effect
on the paper support, on gum arabic, and on iron gall ink.

Introduction

Although many fungicides have been introduced into the paper,
book, and archives conservation field in recent years, thymol
continues to be the best known and is still widely used in
fumigation and humidification chambers and in starch paste
adhesives.

This paper proposes, first, to discuss the structure, uses, and
hazards of thymol; secondly, to investigate the effects of thymol on
paper, pigments, and media as recounted in conservation literature;
and, finally, to describe an experiment designed to test the aging
properties of some papers, pigments, and media which were exposed to
thymol in a controlled situation.

The Structure, Uses, and Hazards of Thymol . Thymol has a
relatively simple organic structure. Its IUPAC name is
5-methyl-2-isopropyl-1-phenol. At room temperature, it is in a
colorless, translucent crystalline or flake form. Thymol has a low
vapor pressure, a property which keeps it from being effective in an
open system and from having any residual fungicidal effect on
objects which have been treated. When used according to appropriate
procedures in a fumigation chamber, thymol can effectively kill
active mold, that is, mold growing on a damp surface. Thymol can
also be used as an inhibitor in what otherwise might be an
environment conducive to the development and growth of mold, such as
in a humidification chamber.

Outside the conservation field, the thymol chamber has had a long
and traditional use in libraries and archives. In many
institutions, artifacts coming into the collection are routinely
placed in the chamber to kill any mold which might be growing on
them. As this paper will attempt to demonstrate, such routine use
of thymol (especially when the artifact is already dry and any mold
is inactive) is, for the most part, useless and may indeed be
harmful to the materials comprising the artifact.

At one time, thymol was considered to be nontoxic, and it is
still in many situations handled cavalierly. Today, however, thymol
is suspected of being a carcinogen and is considered to be
moderately toxic. Precautions should be taken when handling thymol:
one ought to wear rubber gloves and to handle it only in a
ventilated system.

The Literature Search. A search of the conservation
literature on this topic revealed that thymol was the only fungicide
mentioned in Sir Hillary Jenkinson's Manual of Archive
Administration published in 1937. Even then, Jenkinson noted
that thymol had, at times, a softening effect on vellum, size, and
glue, but concluded that "the use of a thymolized duster can do
little harm."

In the past three decades, various controlled experiments, as
well as routine examinations of paper objects have resulted in the
publication of articles indicating that thymol causes discoloration
in paper, parchment, and organic pigments. Ultraviolet light,
elevated temperatures, and/or high levels of relative humidity
accelerate the observed effects considerably.

What is the mechanism by which works of art on paper discolor
after exposure to thymol? One paper dealing specifically with this
topic appeared in 1986 in Studies in Conservation.
Vincent Daniels, of the Conservation Research Section of the British
Museum, and Brian Boyd, of the City of Bristol Museum and Art
Gallery, reported on the "Yellowing of Thymol in the Display of
Prints." In the Bristol Museum, some prints had been sealed in a
framing system with thymol. After three and a half years, it was
revealed that the prints had yellowed.

The authors surmised that since phenols oxidize relatively
rapidly, that the discoloration present in the paper must be
oxidized thymol. There are no reports in the chemical literature
mentioning the identity of the photochemical oxidation product of
thymol, but thymol can be chemically oxidized. Mass spectroscopic
analysis of the discolored thymol provided little evidence for the
oxidation of thymol. Rather, molecular mass peaks indicated that
thymol itself was present, as well as a polymeric form of thymol,
two to five monomer units long. The paper mentioned no further
instrumental analysis to confirm the presence or absence of the
projected photooxidation products. Daniels and Boyd concluded that
only thymol and its polymer were present in the discolored
paper.

Although this article did not deal with fumigation per se,
it is one of the few published papers addressing specifically the
phenomenon of the discoloration of paper in relationship to its
exposure to thymol.

The Experiment

To investigate this area further, an experiment was devised to
examine the effects of the aging characteristics of paper exposed to
thymol. Variables were kept to a minimum. The following six papers
were tested:

A printed page from a late nineteenth-century book

A mold-damaged marbled end paper from the same printed book.
(This was not included for actual fumigation as the mold was
inactive.)

A page from an early nineteenth-century manuscript written in
iron gall ink

Newsprint

Whatman #2 filter paper

A Fabriano handmade rag watercolor paper surface sized with
gelatin to which had been added alum.

On the Fabriano paper were painted twenty-two strips of Winsor
& Newton watercolors. Among these were included four organic
and organo-metallic pigments such as are known to be sensitive to
thymol: gamboge, rose madder, Prussian blue, and indigo. In
addition to these twenty-two were added two other organic pigments
bound in gum arabic: Indian yellow lake and Indian red lake. Also
included were pure gum arabic and wheat starch paste. The gum
arabic was chosen to test the sensitivity of the binder without
pigment. The wheat starch paste was chosen because not only is it
commonly used in paper conservation, but it is also regularly stored
in the presence of thymol.

All of the papers were divided into sections for testing.

Samples of each of the "new" papers, the newsprint, the filter
paper, and the Fabriano paper, were aged in a dry oven at 110°C for
five days. The remaining three papers were considered to be
naturally aged.

Samples of all the papers, both artificially and naturally aged,
were exposed to thymol. The procedure and experimental
specifications for fumigation followed the published guidelines.
An armature of PVC pipe was constructed. From this, the samples
were hung on linen thread. The entire assemblage was placed into a
fume hood. Three-tenths of an ounce of thymol, the proportionate
amount necessary for the space in the chamber (approximately four
cubic feet), was measured into a petri dish and was placed over a
25-watt light bulb. The armature, samples, thymol, and bulb were
covered with a sheet of polypropylene and sealed with tape.

The bulb was turned on for three hours per day for four days in a
row. The heat provided by the bulb accelerated the sublimation of
the thymol. The chamber remained sealed for another three days. At
the end of the week of fumigation, the polypropylene sheet, the
light bulb, and the petri dish were removed. Published instructions
concerning the removal of artifacts from a thymol chamber vary from
no instructions at all to allowing the objects to be in a ventilated
system for several hours. The purpose of the latter directive is to
allow for any residual thymol to sublime so that there is no risk to
the operator in handling the fumigated artifacts. The samples were
allowed to air for a week in the fume hood with the fan on. This
excessive precaution was taken to assure that the paper would be as
free of residual thymol as possible. At the end of the week,
samples from all six of the papers exposed to thymol were returned
to the oven and aged as before. Also aged were samples which had
been previously aged but not exposed to thymol.

At the end of the experimental procedure, there were five test
samples from each of the new papers:

the control

a sample, aged once

a sample, aged, then exposed to thymol

a sample, aged, exposed to thymol, and aged again, and

a sample aged twice, without being exposed to thymol.

The nineteenth-century papers had been divided into four test
strips:

the control, considered to be naturally aged

a sample, exposed to thymol

a sample, exposed to thymol and subsequently aged, and

a sample, "naturally aged", and then artificially aged,
without being exposed to thymol.

The Results of the Experiment

The paper support. A qualitative comparison of these
papers produced, in some cases, rather predictable results. First,
exposure to thymol alone, without aging, appeared to have no visible
effect on the paper, pigments, or media. Aging alone resulted in
discoloration and degradation of the paper and some media.
Introduction of thymol into the aging procedure appeared to
accelerate the degradative process.

When newsprint samples were examined, the deleterious effect of
exposure to thymol appeared in the relatively greater darkening of
the sample which had been exposed to the fungicide and subsequently
aged compared with that sample which had only been aged twice. The
appearance of the other paper samples paralleled the newsprint,
although the differences in color were not so dramatic.

The result of the test of the Whatman filter paper, a high
alpha-cellulose paper with no sizing or fillers, was among the most
revealing. In visible light, the relative darkening of the test
samples paralleled the newsprint. However, when the samples were
viewed under ultraviolet light, the degradation--apparently of the
cellulose--was dramatically revealed. The sample which had been
exposed to thymol and then aged was much brighter than any of the
other samples. As there are no fillers or sizes present, the
brightness of the ultraviolet-produced visible fluorescence is
directly proportional to the breakdown of the cellulose chains.
While aging alone does appear to affect this paper slightly,
exposure to thymol obviously accelerated the degradative
process.

Although the answer is not available at this point, the question
should be raised: What caused the increased yellowing seen in the
samples which were treated and subsequently aged compared with those
left untreated and aged? Was it actually the accelerated
degradation of the cellulose in the filter paper, and/or the size,
in the other samples? Or was it residual thymol or the polymeric or
oxidized form of thymol which had chemically and/or physically
bonded with the paper? Even though the paper samples were
maintained in a ventilated system for a week after exposure to
thymol--far beyond the published specifications which assure
complete sublimation of the chemical --there is still no assurance
that all of the thymol or its oxidation products had left the
paper.

The pigments. Conservation literature provides
documentation of organic pigments' reacting to thymol in treatments.
In this particular experiment, there was no evidence of change in
any of the organic pigments. In visible light, change was evident
only in the zinc white, which acquired a halo. This is found in
naturally-aged watercolors. These test results would demonstrate
only that the halo was a result of aging alone; the formation of a
halo was not a result of the thymol treatment.

When the organic pigment, rose madder, was viewed under
ultraviolet light, it appeared to have been affected by the aging
and further affected by the exposure to thymol. Evaluation of this
physical evidence indicated that this phenomenon was not a reaction
of the lake pigment to the thymol, but rather, it was a response to
the treatment of the binder around the pigment and the paper
beneath. The fact that the red lake showed no change in the visible
light, and that it is among the most transparent of pigments would
support this conclusion.

The media. Further evidence lay in examination of the
binder itself, gum arabic. The strip bearing the gum arabic alone,
without any pigment, had the characteristic appearance of a degraded
film. The strip yellowed in proportion to the amount of time that
it was aged; that section exposed to thymol and subsequently aged
appeared to have yellowed even more. The deterioration of the gum
arabic was even more evident in ultraviolet light. The aged film
fluoresced; the film exposed to thymol and then aged fluoresced even
more brightly. On a molecular level, the reaction which gum arabic
undergoes in dry artificial aging appears to involve cross-linking
and chain breakdown and reforming. It is believed that the hydroxyl
and carboxyl groups react to form ester linkages. The degradation
of the gum arabic, it would then appear, was masked by all but the
most transparent pigment.

Although rarely used as a binder (such as in paste papers), the
reaction of the wheat starch paste is best discussed in this
section. The wheat starch paste manifested the same tendencies as
the watercolor medium. It should be made clear that, in this
particular experiment, the wheat starch paste was taken from the
prepared supply in the lab. This was stored in a container in which
thymol had been allowed to vaporize in order to control the growth
of mold in the paste. In other words, in this particular
experiment, there was no example of wheat starch paste, artificially
aged, which had not been exposed to thymol. The degradation
manifested by the aged samples relative to the others is therefore
inconclusive.

The final medium indicating any response to exposure to the
thymol treatment is iron gall ink. The ink had been applied to a
lightweight laid paper, which, like the Fabriano paper, is of rag
content having a gelatin/alum size. Examination of the sample
exposed to thymol and aged revealed that the ink on each side of the
sample had caused marked discoloration of the paper on the opposite
side. There was no such reaction with the sample that had simply
been aged twice. Here again, as before, the mechanism by which this
degradation takes place is unclear. Exposure to thymol has caused
the paper in contact with the ink and/or the ink and/or perhaps
residual thymol in contact with the ink to oxidize forming increased
discoloration of the paper fibers in close proximity to the ink.

Conclusions

Any conclusions drawn must be qualified by acknowledging that
there are many variables not addressed in this experiment: only a
limited number of papers, sizes, and binders were tested. There
were neither temperature nor relative humidity variables in the
artificial aging process. Not addressed was the question of thymol
and its effect of the alkalinity of paper, and hence, on its effect
on various pigments. Also not addressed were light aging and
photochemical degradation. Acknowledging that this simple
experiment is in no way a definitive study, it is yet possible to
draw some conclusions.

Apparent degradation of the paper support, watercolor binder, and
iron gall ink indicates that exposure to thymol damages their
structures. The exact mechanism of the degradative process is
unclear. Whether the thymol, or some derivation thereof, is
retained in the paper is moot; a change does occur which is damaging
to the object.

Alternatives to thymol are available, and if at all possible,
they should be used. In terms of destroying mold, airing in
sunlight and bathing in alcohol are two options available to the
conservator that do not require elaborate equipment. Vacuum
aspiration appears to remove active mold without leaving the paper
damaged. Thymol should never be employed as a fungal retardant in
a long-term storage context. Thymol should never be considered to
have any residual fungicidal effect. Any conservator faced with
treating active mold on artifacts should be apprised of the
benefits, hazards, and residual effects of any option chosen. The
above investigation would indicate that, if the use of thymol can be
avoided, it should be.

Acknowledgments

For aiding me in the preparation of this paper, I would like to
acknowledge the Art Conservation faculty of Buffalo State College as
well as Elizabeth Kaiser Schulte of the Conservation Center for Art
and Historic Artifacts in Philadelphia. Special thanks go to Cathy
Baker and Dan Kushel, faculty members, who gave so freely of their
time and expertise.

10. Rutherford J. Gettens and George L. Stout, Painting
Materials: A Short Encyclopedia. (New York: Dover
Publications, Inc., 1966), pp. 28-29; John S. Mills and Raymond
White, The Organic Chemistry of Museum Objects (London:
Butterworths, 1987), pp. 66-67; and Jill Sterrett, "Gum Arabic as a
Binder in Watercolor Paints and a Preliminary Study into the Factors
that May Induce its Insolubility in Water," Senior Research Project,
Art Conservation Department, State University College at Buffalo,
Cooperstown, NY, August 1986.

11. Sterrett, "Gum Arabic," Conclusion.

12. Mary Wood Lee, "Alternatives to Fumigation: A Review of
Techniques for the Removal of Mold Growth from Works of Art on
Paper," paper presented at the 16th Annual Meeting of the American
Institute for Conservation, New Orleans, 1-5 June 1988; Lee,"
Removal of Active Mold Growth and Treatment of Structural Damage in
Nine Erotic Indian Miniatures," Preprints of Papers Presented
at the 11th Annual Meeting of the American Institute for
Conservation, Baltimore, 25-29 May 1983, pp. 140-49.

An outline of a thymol fumigation procedure is given. Included
is a description and diagram for a thymol chamber.

Collis, Ivor P. "The Use of Thymol for Document Fumigation."
Journal of the Society of Archivists 4:1 (1970):
53-54.

The use of thymol as a fungicide for bulk treatments is
discussed. The chamber, equipment, and precautions used are
described.

Daniels, Vincent, and Boyd, Brian. "The Yellowing of Thymol in
the Display of Prints." Studies in Conservation 31:4
(Nov. 1986): 156-58.

Experimental work was done to determine the cause for rapid
yellowing of prints displayed in frames containing thymol. Mass
spectroscopic analysis revealed the presence of a polymer thymol;
there was no evidence for the expected photoxidation products.

The author found that after some weeks, thymol could cause
visible destructive effects in sensitive materials. Humidity,
ultraviolet light, and elevated temperatures accelerated the
observed effects considerably.

In the course of a larger study, thymol and other fungicides were
tested. It was demonstrated that thymol has no residual effect.
Samples exposed to thymol exhibited a decrease in mechanical
strength.

Lee, Mary Wood. "Alternatives to Fumigation: A Review of
Techniques for the Removal of Mold Growth from Works of Art on
Paper." A paper presented at the 16th annual meeting of The American
Institute for Conservation, New Orleans, 1-5 June 1988.

The paper discussed various small tools and techniques used in
the process of mold removal, including the construction and use of
the vacuum aspirator.

________, "Removal of Active Mold Growth and Treatment of
Structural Damage in Nine Erotic Indian Miniatures." Preprints
of Papers Presented at the 11th Annual Meeting of the American
Institute for Conservation, Baltimore, 25-29 May 1983, pp.
140-49.

The paper discusses the successful use of a vacuum aspirator to
remove active mold.

The authors investigated the chemical, mechanical, and biological
resistance of paper treated with fungicides, including thymol mixed
with mercuric chloride. The effectiveness of the fungicides on mold
and on the pH of the paper is documented.

Sterrett, Jill. "Gum Arabic as a Binder in Watercolor Paints and
a Preliminary Study into the Factors that May Induce its
Insolubility in Water." Senior Research Project, Art Conservation
Department, State University College at Buffalo, Cooperstown, NY,
August, 1986.

The structure and the possible aging mechanism of gum arabic
binder are discussed.